Probiotics have grown into a nearly $16 billion global industry; with more and more American companies adding so-called "friendly" bacteria to products from yogurt to cereal.

The claim is that probiotics improve your digestive system, boost your immunity, and even ward off the flu. Now, a new study launched by a Danish company that makes probiotic products says kids on probiotics had fewer flu symptoms than kids who did not over the course of one winter.

In that study, only 18 percent of kids taking two kinds of probiotics had a fever over a six-month period, versus 66 percent of the kids taking none; and only one-third of the probiotics group had a cough or runny nose, versus more than 80 percent of the kids taking nothing.

Dr. Tasneem Bhatia at the Atlanta Center for Holistic and Integrative Medicine says, "The majority of our immunity is actually centered in our gut. That's why it's not very surprising to me that we're seeing a role for probiotics in the prevention of flu and upper respiratory infections."

Some doctors caution the new study, ordered up by a company with a financial stake in its success, only tested two specific strains on Children in china. Additional studies have also found illness-fighting benefits in probiotics, but other studies have questioned whether they have any value.

Doctors are calling for more independent research and say this flu season, parents should still focus on the basics.

"Namely, getting vaccinated, washing hands, avoiding people that are sick," said Dr. William Shaffner at Vanderbilt University. "And, if you're sick yourself or your child is sick, keeping that child at home."